5091-1596E.pdf

Agilent

High Accuracy and

Fast RF Inductor Testing

Application Note 369-10

Agilent 4285A

Precision LCR Meters

Introduction

Problems and the solutions offered

by the Agilent 4285A

RF inductors are employed in electronic equipment such

as VCRs and intermediate frequency (IF) circuits for

AM/FM radios, mobile radios, pagers and code-less

telephone, which require RF inductors to be smaller,

lower cost, and have higher reliability. This application

note describes solutions offered by the Agilent 4285A

precision LCR meter for realizing these requirements.

Information for accurate and fast RF inductor testing,

and for practical simple test systems are discussed.

1. Measurement accuracy and measurement speed

Stable inductance (L) and quality factor (Q) low

inductance measurements are difficult to make, because

measuring low inductance values means measuring low

impedance, values. If a higher measurement frequency

(MHz) is issued to increase the measured impedance,

the LCR meter’s measurement accuracy is worse,

so accurate measurement data cannot be obtained.

The 4285A can perform reliable and fast measurements

with 0.5% inductance measurement accuracy even in the

MHz frequency range, and with a 30 ms measurement

speed. Formerly LCR meters could not measure

accurately enough for testing high Q inductors, and

Q meters required long measurement time and high

operator skill. Table 1 lists the 4285A’s key specifications.

2. Measurement condition setup

There are many types of RF inductors, so the measurement

frequency range for an inductor is selected from

across a frequency range spanning kHz through MHz,

depending on the inductance value to be measured and

the application in which it will be used. The 4285A

covers a measurement frequency range of from 75 kHz

to 30 MHz, including frequencies defined in the MIL

C-15305D standard (79.6 kHz, 252 kHz, 796 kHz, ..25.5 MHz)

A constant current test signal gives the best inductance

measurement results, because inductive devices are

current mode devices and L and Q values depend on

current levels rather than voltage levels. Q meters do

not offer current test signals, and former LCR meters

used voltage test signals, not current test signals.

The 4285A offers both voltage and current test signal

level selections. Furthermore, its auto level control (ALC)

function allows constant test signal level measurements.

Table 1. 4285A key specifications

Agilent 4285A (standard)

Measurement method

Auto balancing bridge

Measurement frequency

75 kHz to 30 MHz with 100 Hz resolution

Basic accuracy (typical)

10 mH

L: ±0.18%

@100 kHz

Q: ±5% @Q=30

1 µH

L: ±0.5%

@10 MHz

Q: ±10% @Q=30

Measurement speed (typical)

30 ms/65 ms/ 200 ms

Test signal level

V: 5 mVrms ~ 2 Vrms

(10 mVrms ~ 1 Vrms in constant mode)

I: 200 µ/Arms ~ 20 mArms

(100 µArms ~ 20 mArms in constant mode)

V/I monitors are available

Example of an accurate and fast

measurement system

3. Functions for a test system

Formerly an LCR meters’ cable extension capability was

limited to an upper measurement frequency limit of

approximately 15 MHz, and they could not fully compensate

for errors due to extension cables and test fixtures. So,

the LCR meter performance was limited when used in a

measurement test system. The 4285A can use 1 m or 2 m

test leads over its full frequency range, and its open/short

/load correction function can compensate for measurement

errors due to extension leads. You can obtain accurate

measurement values by performing error correction at

the actual measurement frequencies (for example the

frequencies defined in MIL standards).

This paragraph describes an example of L-Q measurement

and sorting system for RF inductor production line

applications, and gives some techniques for designing

such systems.

Figure 1 shows a simplified model of an RF inductor

test system. In this system, the 4285A measures L and Q

and performs BIN sorting using its internal comparator.

Equipment used in this system is as follows (refer to

‘Appendix A. Ordering Information’ for details).

• 4285A precision LCR meter with option 201 handler

interface

• 16048A test leads (l m, BNC connectors )

• Handler interface cable (Amphenol 36 pin connectors

(male))

• Automatic component handler (compatible with

4285A option 201)

An internal comparator and an optional automatic

handler interface are available for setting up RF inductor

test systems.

The following are technical hints for designing a test

system, and for maximizing the 4285A’s performance.

1. Cable connections

Use Agilent test leads to extend the measurement

terminals of the 4285A to the handler’s contact

electrodes.

You can select 1 m or 2 m test leads, however, 1 m test

leads are recommended to minimize the error caused

by the addition of extension cables.

2. Extension leads and contact electrodes

• The 16048A test leads will extend the signal path

from the 4285A measurement terminals to the end

of cables while maintaining a Four-Terminal Pair

configuration.

Figure 1 . Example of an accurate and

fast L-Q measurement system

Use connector plate (Agilent PN 16032-60001,

furnished with the 16048A) to change the terminal

configuration and extend the leads to the contact

electrodes as shown in figure 2.

Figure 2 . Contact electrodes

Extension leads and contact electrodes which are not a

Four-Terminal Pair configuration cause measurement

errors, because of residual impedance and noise. Thus,

pay close attention to the following items. Especially

when performing low inductance measurements, the

following must be closely considered, because for low

impedance measurements, measurement repeatability

depends on contact electrode stability rather than the

4285A’s inductance measurement repeatability.

• To shield against noise, the outer conductors of the

extension leads must be connected to each other at

a point as near as possible to the test device.

• To reduce residual impedance, the extension leads

must be as short as possible.

As shown in figure 4, leave the contact electrodes open,

then perform an open correction measurement to

compensate for the stray admittance of extension leads

and the contact electrodes.

The short correction function compensates for the

residual impedance of extension leads and contact

electrodes, and is very important for L and Q

measurements. Short the electrodes to each other as

directly as possible, then perform a short correction

measurement. If you want to base your measurement on

a working standard as a reference, then we recommend

performing a load correction measurement in addition

to an open/short correction measurement. The load

correction function compensates for measurement errors

of a test device by using load correction coefficients

obtained from the measured values and the reference

values of a working standard (load). Input the reference

values of the working standard such as; L and Q values

(C and D in figure 3). Connect the working standard

to the contact electrodes, then perform the load

correction measurement.

For stable residual impedance, the layout, positioning,

and length of the extension leads and contact electrodes

must not be changed, and careful consideration must

be given to eliminating mechanical vibration.

3.Error correction

The Agilent 4285A’s powerful error correction function

can be used to compensate for the residual impedance

of extension leads and contact electrodes to give more

accurate impedance (L and Q) measurement values.

When using the 4285A, the reference values of the

working standard can be obtained at the actual test

frequency and signal level. Measure the working standard

using the 4285A with a test fixture (such as the, 16047E

or 16034G) setup under actual measurement conditions,

and input these values as the reference values.

First, set the cable length function to 1 m to correct

for the error caused by 1 m test leads (A in figure 3).

Open/short/load correction should be performed

at the actual measurement frequencies. Up to seven

correction frequencies can be set (B in figure 3), so

the six frequency points specified in MIL C-15305D

can be covered.

Figure 3. Correction function setup

Figure 4. Performing correction measurements

4. Measurement conditions and comparator setup

Figure 6 shows a sample timing chart for an interface.

The total testing time for a device is as follows.

The measurement conditions, including frequency and

signal level, should be set to appropriate values.

Measurement Time (30 ms @SHORT INTEG TIME) -

Calculation/Comparison time + Device change/Sort Time.

If necessary, a delay time (l ms - 60 s) can be inserted

before the measurement time (Tm1 in figure 6) to allow

for a mechanical contact check within the handler.

Figure 5 shows a comparator setup to sort test devices

into BIN 1 through BIN 9 by L value and into IN/OUT

by Q value.

6. Memory card

5. Interface timing design

Setup data including measurement conditions and

comparator limit values can be stored in a memory

card. Using this function, all setup data can be recalled

with just a few key strokes, this is especially useful on the

production line.

The 4285A option 201 handler interface has the capability

for adjusting the control signal timing between the 4285A

and a handler. The interface is triggered by an

(/EXT_TRIG) signal from a handler, and outputs an end

of analog measurement signal (/INDEX), an end of

digital computation and comparison signal (/EOM),

and BIN sort data (/BIN 1 through/BIN 9). Each output

and input is isolated using an optocoupler, and the

outputs are open collector.

Figure 5 : Comparator setup

Figure 6 : Sample interface timing chart

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